The Plesiochronous Digital Hierarchy (PDH)

Until the late 1950s the telephone system consisted almost entirely of analogue transmission lines. Trunk lines between exchanges carried multiple voice channels simultaneously using frequency division multiplexing (FDM). This entailed the use of expensive modulators, demodulators and filters for each voice channel, and commercial pressures created a need to develop more cost-effective exchange equipment. The digitisation of an analogue voice channel into a 64 kbps digital channel (designated as digital signalling level zero or simply DS0) using pulse code modulation (PCM) made it possible to use time division multiplexing (TDM) to multiplex a number of voice channels onto a trunk line known as a T-carrier in North America and Japan (the European equivalent used in most of the rest of the world is called an E-carrier). T1 lines were originally four-pair copper wire or coaxial cables, but these have largely been superseded by optical fibre.

The T1 carrier originally carried 24 digital voice channels, each with a bit rate of 64 kbps. Each TDM frame on the T1 line carried 24 bytes of voice data (8 bits per voice channel) plus a single framing bit to facilitate synchronisation and de-multiplexing by the receiver. Since each byte of data represents a 125μs voice sample, the required frame rate was 8,000 frames per second. The total bit rate for a T1 line is therefore 8,000 x ((24 x 8) + 1) = 1,544,000 bits per second (1.544 Mbps) and is designated as digital signalling level 1 or DS1. The E-carrier system developed by the European Conference of Postal and Telecommunications Administrations (CEPT) benefitted from lessons learned during the development of the T-carrier technology, refining and improving on the earlier system. Like the DS0, the basic E0 voice channel has a bit rate of 64 kbps. An E1 carrier, however, has a total of 32 timeslots per frame. Each timeslot consists of only 8 bits (i.e. one timeslot per 8-bit channel), giving a total bit rate of 8,000 x 32 x 8 = 2,048,000 bits per second (2.048 Mbps). One timeslot (TS0) is reserved for framing, while a second timeslot (TS16) may be used for signalling purposes (i.e. to control call setup and termination).

The T1 frame format (FB = framing bit)

The E1 frame format (timeslots 1-15 and 17-31 are used for data)

The T1 and E1 carriers are the first level of multiplexing in the Plesiochronous Digital Hierarchy (PDH). Four first level carriers are multiplexed together to create a higher capacity second level carrier. Each subsequent level in this digital hierarchy multiplexes together four or more carriers from the level below (the only exception is the intermediate level DS1C between DS1 and DS2 in the North American system). The bit rates for each level of the North American, European and Japanese Plesiochronous digital hierarchies are shown in the table below.

PDH Levels and Bit Rates

Carrier level

North America

Europe

Japan

Voice/data channel

64 kbps (DS0)

64 kbps (E0)

64 kbps

First level

1.544 Mbps (DS1)24 channels

2.048 Mbps (E1)32 channels

1.544 Mbps24 channels

Intermediate level

3.152 Mbps (DS1C)48 channels

N/A

N/A

Second level

6.312 Mbps (DS2)96 channels

8.448 Mbps (E2)128 channels

6.312 Mbps96 channelsor7.786 Mbps120 channels

Third level

44.736 Mbps (DS3)672 channels

34.368 Mbps (E3)512 channels

32.064 Mbps480 channels

Fourth level

274.176 Mbps (DS4)4032 channels

139.264 Mbps (E4)2048 channels

97.728 Mbps1440 channels

Fifth level

400.352 Mbps (DS5)5760 channels

565.148 Mbps (E5)8192 channels

565.148 Mbps8192 channels

The European hierarchy increases the number of channels by a factor of four at each higher level, but if you look at the gross bit rates for each level you will observe that it is actually significantly greater than four times the bit rate of the level below it. This is because a certain amount of additional overhead is required in order to ensure that multiplexing and de-multiplexing can be carried out successfully. The plesiochronous nature of the hierarchy (from the Greek plesio meaning near and chronos meaning time) means that bit streams from different sources are not guaranteed to have exactly the same timing, even though the nominal bit rate is the same. This is due to the fact that each piece of exchange equipment uses its own internal clock for timing purposes. Although these clocks are extremely accurate they are not synchronised, and there will be enough difference in the actual bit-rates of the tributaries feeding into a given multiplexing level to require some additional overhead in order to provide the necessary synchronisation.

The multiplexer hierarchy for the European version of PDH

Assume (for the purposes of this discussion) that the multiplexer is multiplexing four incoming data streams onto a single carrier. The nominal data rate of the carrier is four times that of any of its four tributaries, although the aggregate data rate of its output is slightly higher than four times the nominal data rate of the tributaries in order for it to be able to deal with incoming signals that might be running at or near the upper bound of the allowed bit rate variance. The multiplexer takes one bit from each incoming date stream in turn for onward transmission (a process known as bit interleaving). The multiplexer initially assumes that each tributary data stream is running at the maximum allowed bit rate, and looks for the next bit from a particular data stream at the appropriate time. Every so often (unless all incoming data streams are indeed running at exactly their maximum allowed speed, which is highly unlikely), the multiplexer will look for an incoming bit that has not yet arrived. If this occurs, a stuffing (or justification) bit is added to the multiplexed data stream in place of an actual data bit to maintain the outgoing bit rate. Additional bits are used to tell the receiving multiplexer/de-multiplexer whether or not stuffing bits have been added to each frame, and if so which bits are the stuffing bits. These stuffing bits, together with the additional bits required to identify them at the receiver, constitute the bit-rate overhead for each level in the hierarchy.

In order that the data streams from different exchanges are not allowed to drift too far apart in terms of their timing, the exchange clocks are synchronised using one of two possible synchronisation strategies. In mutual synchronisation each exchange clock is allowed to drift within a certain tolerance, but the clocks are interconnected and the mean frequency of all of the clocks provides a benchmark to which all of the clocks periodically adjust themselves. The mean frequency itself is kept within specified tolerances with reference to a primary reference clock (PRC) that conforms to the ITU-T G.811 specification. The second strategy that may be used is called master-slave synchronisation, in which each exchange clock at any given level of synchronisation is synchronised to the exchange clock at the next highest level. The highest level exchange clock is synchronised to a master clock that provides timing information for the entire network.

The use of bit interleaving, and the presence of stuffing bits in the multiplexed data stream, means that it is virtually impossible to extract data belonging to an individual channel or tributary without de-multiplexing the high-level data stream down to the required level. To extract a single 64 kbps channel from a 565.148 Mbps trunk, the complete hierarchy of de-multiplexers is required as shown in the illustration below. The same principle applies to inserting a channel or tributary into a multiplexed data stream.

The full multiplexer hierarchy must be used to drop or add a 64kbps channel

PDH made little provision for management of the network, and the need to fully de-multiplex a high level carrier to extract a lower level signal meant that increasing the capacity of PDH networks beyond a certain point was not economically viable. The main economic factor was the cost of the equipment required at each cross-connect point within the network where either individual channels or low-level multiplexed data streams might need to be extracted or added. It also added additional latency and increased the possibility of errors occurring, thereby reducing network reliability. Matters were further complicated by the fact that much of the lower-level transmission lines were still copper-based, which meant that optical line terminating equipment (OLTE) had to be used to provide the interface between those parts of the network based on copper and those based on optical fibre. The lack of standardisation, both between PDH networks operating in different parts of the world and between competing equipment manufacturers, severely hampered interoperability and necessitated the use of expensive conversion equipment between networks operating in Europe, North America and Japan. Increasing demand for network bandwidth virtually dictated the use of synchronous optical technologies that were both standardised and scalable, and led to the development of the Synchronous Optical Network (SONET) in North America and the Synchronous Digital Hierarchy (SDH) in Europe and the rest of the world.